1. Field of the Invention
The present invention relates to a method and apparatus for calibrating measuring machines and more particularly to a method and apparatus suitable for calibrating surface texture measuring machines such as three-dimensional measuring machines and the like.
2. Description of the Related Art
A three-dimensional measuring machine for measuring a three-dimensional shape of a work; a contour measuring machine and an image measuring machine for measuring a two-dimensional contour; a roundness measuring machine for measuring roundness; and a surface roughness measuring machine for measuring waviness and roughness of a work surface have been known as surface texture measuring machines that can be employed to measure surface shapes of works, such as contour, roughness and waviness. These machines are generally equipped with a guide mechanism for moving a contact or non-contact sensor relative to a work in one- or multi-axial arrangement.
These guide mechanisms commonly include a guide, an feed screw and a nut for mating with the screw to move a slider that is coupled to the nut. A linear scale is employed, for example, to measure a movement of the slider. There is another type of guide mechanism that is not always equipped with an feed screw but consists only of a guide and a slider. This guide mechanism employs a linear scale to read an amount of displacement of the slider manually moved. The slider is commonly provided with a sensor such as a touch probe and a CCD camera attached thereon.
Environmental pollution, environmental variation-related deformations and other errors are inevitably caused in these guide mechanisms. As a result, the slider can not move correctly and may give errors in data measured from a work by the sensor located on the slider.
For example, of the above guide mechanisms, in a straight guide mechanism designed for the purpose of straight movement, various errors can be considered: a straightness error in a vertical plane; a straightness error in a horizontal plans; a pitching error; a rolling error; a yawing error; and an indication error on the linear scale itself.
Of the surface texture measuring machines, as a three-dimensional Coordinate Measuring Machine (hereinafter referred to as CMM) has a structure that includes three sets of such straight guide mechanisms intersecting at right angles with each other, orthogonal errors occur between the straight guide mechanisms additionally. Therefore, at least 21 types of geometrical deviations in total may possibly occur in such the CMM.
As a result, a great effort is required disadvantageously in an operation to strictly calibrate such the surface texture measuring machines.
For example, a measuring machine designed for the purpose of calibrating a geometrical deviation of the CMM is currently limited from the viewpoint of the variety of measurement methods while it has been employed long in history. In many cases, the mainstream is a measurement instrument for mono-functionally detecting a geometrical deviation, for example, a laser interferometer and an electrical level. To manage uncertainty in measurement using the measuring machines, it is required to handle the machine and perform alignment prior to every measurement by an operation-learned operator. As a result, it is required to perform calibration by a skilled worker spending many hours, resulting in a high-cost, labor-intensive work step that can not expect a saving in labor. On the other hand, when the geometric accuracy by the current CMM is standardized within its operable range, it has already reached several ppm. Thus, it is difficult to realize such a calibration method that can be satisfied from the viewpoint of uncertainty in view of simply trying automation.
Reflecting the recent high concerns on traceability and uncertainty in calibration, a trend can be found in an offer of a geometric calibration to the user for an appropriate market price and quality. In such the case, it can not be expected to ensure a calibrating operator with extremely high techniques. Even if it can be expected, the user""s satisfaction from the viewpoint of cost remains low. More importantly, the geometric calibration in the market aims at an additional calibration, which is performed to a measuring machine already calibrated generally by the maker using some method, to issue an official certificate of calibration on uncertainty in calibration. Therefore, in the case of the CMM, it is not required to measure a measurement space including everything. In addition, it is possible to evaluate at a considerably long interval between measurement points. With this regard, it has a characteristic of the sampling test.
To the contrary, the calibration in the process of manufacturing CMMs has a different property from that in the market. First, as the object is a CMM that is not calibrated previously in history, it is required to locate measurement points that can cover the whole measurement space at a necessarily and sufficiently fine interval. This corresponds to a 100% and full-function inspection. In addition, a premise lies in compensating the geometric deviation of CMM using the calibration result. Therefore, it is required to adopt a calibration method that provides a calibrated value as the geometric deviation kinematically described usable for compensation of precision. Due to such the property, the dependency on the learned worker is particularly higher compared to the commercial calibration laboratory, presenting a high barrier against saving in labor.
The present invention has been made to solve the above disadvantages and accordingly has a first object to provide a calibration method and apparatus capable of achieving automated calibration.
The present invention has a second object to provide a calibration method and apparatus capable of increasing precision of spherical parameter assumption to improve precision of calibration.
To achieve the above objects, the present invention is provided with a first calibration method, comprising the steps of: positioning a reference measuring machine previously calibrated and an object measuring machine to be calibrated in such a manner that a measurement space by the reference measuring machine is superimposed on a measurement space by the object measuring machine; acquiring first measurement values by the object measuring machine and second measurement values by the reference measuring machine each on plural points in the measurement spaces respectively by the reference measuring machine and the object measuring machine; and calibrating the object measuring machine based on the first and second measurement values.
The present invention is also provided with a calibration apparatus, which is an apparatus for calibrating measuring machines to calibrate an object measuring machine to be calibrated, comprising: a reference measuring machine previously calibrated and positioned in such a manner that a measurement space by the reference measuring machine is superimposed on a measurement space by the object measuring machine; and computing means, using first measurement values obtained by the object measuring machine and second measurement values obtained by the reference measuring machine each on plural points in the measurement spaces respectively by the reference measuring machine and the object measuring machine, for calibrating the object measuring machine based on the first and second measurement values.
In an embodiment of the present invention, the object measuring machine includes: a first base plate for mounting a work thereon; a first guide mechanism consisting of a first guide and a first slider located on the basis of the first base plate; a first scale for providing a first coordinate signal on detection of a movement of the first slider relative to the first guide; and a first sensor located on the first slider for providing a first detection signal, the object measuring machine processing at least one of the first detection signal and the first coordinate signal to measure a surface texture on the work. The reference measuring machine includes: a second base plate for mounting the object measuring machine thereon; a second guide mechanism consisting of a second guide and a second slider located on the basis of the second base plate; a second scale for providing a second coordinate signal on detection of a movement of the second slider relative to the second guide; and a second sensor located on the second slider for providing a second detection signal. One of the first and second sliders has a reference device located thereon. The computing means, on detection of the reference device by the sensor located on the other of the first and second sliders, takes the first and second coordinate signals as the first and second measurement values to calibrate the first coordinate signal based on the first and second coordinate signals.
Preferably, the reference device includes a support member and a sphere supported on the support member. Preferably, the reference device includes three spheres supported on the support member and not arrayed in line. Preferably, the first or second sensor includes a spherical probe. Preferably, the sphere has a diameter almost similar to that of the probe. Preferably, the sphere has six measurement points uniformly distributed on the spherical surface.
In another embodiment of, the present invention, at least one of the first and second scales is equipped with a temperature sensor to provide an output for correcting at least one of the first and second coordinate signals.
In a further embodiment of the present invention, the second base plate further includes a displacement gauge for detecting a displacement of the first base plate relative to the second base plate.
In a yet further embodiment of the present invention, the object measuring machine includes: a first base plate for mounting a work thereon; a first guide mechanism consisting of a first guide and a first slider located on the basis of the first base plate; a first scale for providing a first coordinate signal on detection of a relative movement of the first slider to the first guide; and a first sensor located on the first slider for providing a first detection signal. The object measuring machine processes at least one of the first detection signal and the first coordinate signal to measure a surface texture on the work. The reference measuring machine includes: a second base plate for mounting the object measuring machine thereon; a second guide mechanism consisting of a second guide and a second slider located on the basis of the second base plate; and a second scale for providing a second coordinate signal on detection of a movement of the second slider relative to the second guide. The apparatus further comprises a coupler for coupling the first slider with the second slider. The computing means calibrates the first coordinate signal based on the second coordinate signal.
Preferably, the coupler includes: a reference coupling member secured on the second slider; an object coupling member secured on the first slider; and a wire for coupling the object coupling member with the reference coupling member to tow one member relative to the other member in the guide direction of the first or second guide mechanism. Preferably, the wire couples the one of the reference coupling member and the object coupling member to the other in such a manner that the other relative to the one can not move in the guide direction of the first or second guide mechanism but can rotate in the rotational direction.
Such the comparative measurement method for spatial coordinates is a method of calibrating coordinate indication values from the object measuring machine with reference to measurement values from the reference measuring machine previously calibrated. This method can be controlled easily from the host computer and does not need manual operations on alignment and so forth by a worker unlike the conventional calibration method. Therefore, it is possible to realize automated calibration. This method gives extremely high efficiency to the geometrical calibrating operation, which has been a high-cost intensive work, for the measuring machine.